BHSU Microorganism Differentiation Questions

Chapter 3Bacteria and Archaea
©McGraw-Hill Education
Form and Function of
Bacteria and Archaea
How bacteria and archaea are different from
eukaryotes:
• The way their DNA is packaged: lack of nucleus and
histones
• The makeup of their cell wall: peptidoglycan and
other unique chemicals
• Their internal structures: lack of membrane-bound
organelles
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The Structure of the Bacterial Cell
All bacterial cells possess:
• Cytoplasmic membrane
• Cytoplasm
• Ribosomes
• Cytoskeleton
• One (or a few) chromosome(s)
Most bacterial cells possess:
• Cell wall
• A surface coating called a glycocalyx
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Structures Found in Some Bacteria
Some, but not all bacterial cells possess:
• Flagella, pili, and fimbriae
• An outer membrane
• Nanowires/nanotubes
• Plasmids
• Inclusions
• Endospores
• Microcompartments
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Structure of a Bacterial Cell
Jump to long description
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Bacterial Shapes and Arrangements
Most bacteria function as independent singlecelled, unicellular organisms:
• Some act as a group in colonies or biofilms
• Some communicate through nanotubes
Bacteria have an average size of 1 μm (micron):
• Cocci: circumference of 1 μm
• Rods: length of 2 μm and a width of 1 μm
Pleomorphism: variation in the size and shape
of cells of a single species due to nutritional and
genetic differences
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Bacterial Shapes
Jump to long description
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Source: CDC/Janice Haney Carr (a); Source: CDC/Janice Haney Carr (b); Source: CDC/Janice Haney Carr (c); Source: USDA/Photo by De Wood. Digital
colorization by Chris Pooley (d); ©VEM/Science Source (e); ©Eye of Science/Science Source (f)
Bacterial Arrangements: Cocci
Arrangement of cocci:
• Single
• Diplococci: pairs
• Tetrads: groups of four
• Staphylococci or micrococci: irregular clusters
• Streptococci: chains
• Sarcina: cubical packet of eight, sixteen, or more
cells
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Arrangement of Cocci
Jump to long description
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Bacterial Arrangements: Bacilli
Arrangement of bacilli:
• Single
• Diplobacilli: pair of cells
with ends attached
• Streptobacilli: chain of
several cells
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©De Agostini/Getty Images
External Structures
Appendages:
• Motility: flagella and axial filaments
• Attachment points or channels: fimbriae, pili, and
nanotubes/nanowires
Flagellum:
• Primary function is motility
• Three distinct parts:
• Filament
• Hook
• Basal body
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Flagellum of a Gram-Negative Cell
Jump to long description
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Sarkar MK1, Paul K, Blair D., “Chemotaxis signaling protein CheY binds to the rotor protein FliN to control the direction of flagellar rotation in Escherichia
coli,” PNAS May 18, 2010 vol. 107 no. 20 9370-9375 (b)
Arrangement of Flagella
Polar arrangement: flagella attached at one or
both ends of the cell
• Monotrichous: single flagellum
• Lophotrichous: small bunches or tufts of flagella
emerging from the same site
• Amphitrichous: flagella at both poles of the cell
Peritrichous arrangement: flagella are dispersed
randomly over the surface of the cell
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Types of Flagellar Arrangements
Jump to long description
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©Science Photo Library/Alamy Stock Photo (a); Source: CDC/ Melissa Brower (b); ©Heather Davies/Science Source (c); ©Smith Collection/Gado/Getty Images (d)
Fine Points of Flagellar Function
Chemotaxis: movement of bacteria in response
to chemical signals:
• Positive chemotaxis: movement toward favorable
chemical stimulus
• Negative chemotaxis: movement away from a
repellant
• Run: rotation of flagellum counterclockwise,
resulting in a smooth linear direction
• Tumble: reversal of the direction of the flagellum,
causing the cell to stop and change course
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Operation of Flagella
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Chemotaxis in Bacteria
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Periplasmic Flagella
Spirochetes: corkscrew-shaped bacteria:
• Possess an unusual, wriggly mode of locomotion
due to periplasmic flagella
Periplasmic flagella:
• Also called axial filaments
• Internal flagellum enclosed in the space between
the cell wall and the cytoplasmic membrane
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Appendages for Attachment or
Channel Formation: Fimbriae
Fimbria/fimbriae:
• Small, bristle-like fibers sprouting off the surface of
many bacterial cells
• Allow tight adhesion between fimbriae and
epithelial cells, allowing bacteria to colonize and
infect host tissues
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©Eye of Science/Science Source
Appendages for Attachment or
Channel Formation: Pili and Nanotubes
Pilus/pili:
• Used in conjugation between bacterial cells
• Well characterized in gram-negative bacteria
• Type IV pilus can transfer genetic material, act like fimbriae
and assist in attachment, and act like flagella and make a
bacterium motile
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Conjugating Process
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©L. Caro/SPL/Science Source
S Layer and Glycocalyx
S layer:
• Single layers of thousands of copies of a single protein
linked together like chain mail
• Only produced when bacteria are in a hostile
environment
Glycocalyx:
• Coating of repeating polysaccharide or glycoprotein
units
• Slime layer: loose, protects against loss of water and
nutrients
• Capsule: more tightly bound, denser, and thicker;
produce a sticky (mucoid) character to colonies on agar
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Position of Bacterial S Layer
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©Russell Kightley/Science Source
Encapsulated Bacteria
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CDC (a); ©Michael Abbey/Science Source (b)
Specialized Functions
of the Glycocalyx
Capsules:
• Formed by many pathogenic bacteria
• Have greater pathogenicity
• Protect against phagocytosis
Biofilms:
• Plaque on teeth protects bacteria from becoming
dislodged
• Responsible for persistent colonization of plastic
catheters, IUDs, metal pacemakers, and other
implanted medical devices
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Biofilm Formation
Jump to long description
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©Scimat/Science Source (b)
The Cell Envelope
Lies outside the cytoplasm
Composed of two or three basic layers that each
perform a distinct function, but together act as a
single protective unit:
• Cell wall
• Cytoplasmic membrane
• Outer membrane (in some bacteria)
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Comparison of Gram-Positive and
Gram-Negative Cell Envelopes
Jump to long description
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©Dr. Kari Lounatmaa/Science Source (gram-positive cell.); ©Dennis Kunkel Microscopy, Inc./Medical Images (gram-negative cell)
The Cell Wall
Helps determine the shape of a bacterium
Provides strong structural support to keep the
bacterium from bursting or collapsing because
of changes in osmotic pressure:
• Certain drugs target the cell wall, disrupting its
integrity and causing cell lysis (disintegration or
rupture) of the cell
Gains its relative rigidity from peptidoglycan
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Peptidoglycan
Compound composed of a repeating framework of long
glycan (sugar) chains cross-linked by short peptide
(protein) fragments
Provides a strong but flexible support framework
Jump to long description
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Gram-Positive Cell Wall
Thick, homogenous sheet of peptidoglycan:
• 20 to 80 nm in thickness
Contains teichoic acid and lipoteichoic acid:
• Function in cell wall maintenance and enlargement
• Contribute to the acidic charge on the cell surface
©McGraw-Hill Education
Gram-Negative Cell Wall
Single, thin sheet of peptidoglycan:
• 1 to 3 nm in thickness
Thinness gives gram-negative cells greater
flexibility and sensitivity to lysis
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Steps in a Gram Stain
Jump to long description
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Nontypical Cell Walls:
Acid-Fast Bacteria
Mycobacterium and Norcardia: contain
peptidoglycan and stain gram-positive, but bulk
of cell wall is composed of unique lipids
Mycolic acid:
• Very-long-chain fatty acid
• Found in the cell walls of acid-fast bacteria
• Contributes to the pathogenicity of the bacteria
• Makes bacteria highly resistant to certain chemicals
and dyes
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Nontypical Cell Walls: Archaea
Some have cell walls composed entirely of
polysaccharides
Others have cell walls made of pure protein
All lack true peptidoglycan structure
Some lack a cell wall entirely
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Mycoplasmas and Other
Cell-Wall-Deficient Bacteria
Mycoplasmas:
• Naturally lack a cell wall
• Sterols in the cell membrane stabilize the cell
against lysis
• Mycoplasma pneumoniae: “walking pneumonia”
L forms:
• Some bacteria that naturally have a cell wall but lose
it during part of their life cycle
• Role in persistent infections
• Resistant to antibiotics
©McGraw-Hill Education
The Gram-Negative Outer Membrane
Similar in composition to most membranes,
except it contains specialized polysaccharides
and proteins
Lipopolysaccharide:
• Signaling molecules and receptors
• Endotoxin
Porin proteins:
• Special membrane channels that only allow certain
chemicals to penetrate
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Cytoplasmic Membrane Structure
A lipid bilayer with proteins embedded
Regulates transport of nutrients and wastes
Selectively permeable: special carrier
mechanisms for passage of most molecules
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Differences in
Cell Envelope Structure
Outer membrane of gram-negative bacteria
contributes an extra barrier:
• Resistant to certain antimicrobial chemicals
• More difficult to inhibit or kill than gram-positive bacteria
Alcohol-based compounds dissolve lipids in the outer
membrane and therefore damage the cell:
• Alcohol swabs used to cleanse the skin before certain
medical procedures
Treatment of infections caused by gram-negative
bacteria requires drugs that can cross the outer
membrane
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The Cytoplasm
70 to 80% water
Complex mixture of sugars, amino acids, and salts
Serves as a pool for building blocks for cell
synthesis or sources of energy
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Bacterial Chromosomes and Plasmids
The hereditary material of most bacteria exists
in the bacterial chromosome
DNA is aggregated in the nucleoid
Plasmids:
• Nonessential pieces of DNA
• Confer protective traits such as drug resistance and
toxin and enzyme production
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Ribosomes
Site of protein synthesis
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Bacterial Ribosome
Jump to long description
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Inclusion Bodies and
Microcompartments
Used for food storage
Pack gas into vesicles for buoyancy
Store crystals of iron oxide with magnetic
properties
Bacterial microcompartments:
• Outer shells made of protein, arranged
geometrically
• Packed full of enzymes designed to work together in
biochemical pathways
©McGraw-Hill Education
The Cytoskeleton
Some bacteria produce long polymers of protein
similar to eukaryotic cells for the cytoskeleton:
• Arranged in helical ribbons around the cell
• Contribute to cell shape
• Have also been identified in archaea
• Unique to non-eukaryotic cells – may be a potential
target for antibiotic development
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Bacterial Endospores
Dormant bodies
Produced by Bacillus,
Clostridia, and Sporosarcina
Vegetative cell: metabolically
active
Sporulation: induced by
environmental conditions
Endospores resist extremes of
heat, drying, freezing,
radiation, and chemicals that
would kill vegetative cells
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©Science Source
Sporulation Process in
Bacillus Species
Jump to long description
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©Science Source
The Medical Significance of
Bacterial Endospores
Bacillus anthracis: agent of anthrax
Clostridium tetani: cause of tetanus
Clostridium perfringens: cause of gas gangrene
Clostridium botulinum: cause of botulism
Clostridium difficile: “C. diff,” a serious
gastrointestinal disease
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Archaea: The Other “Prokaryotes”
Considered a third cell type in a separate
superkingdom
More closely related to domain Eukarya than
bacteria:
• Share rRNA sequences not found in bacteria
• Protein synthesis and ribosomal subunit structures
are similar
©McGraw-Hill Education
Archaea Differ from Other Cell Types
Extremophiles:
• Some live at extremely high or low temperatures
• Some need extremely high salt or acid concentrations to survive
• Some live on sulfur or methane
Some live on the human body and may be capable of
causing human disease
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Comparison of Three Cellular Domains
Characteristic
Bacteria
Chromosomes
Single or a few, circular Single, circular
Types of ribosomes
70S
70S but structure is
similar to 80S
80S
Contains unique
ribosomal RNA
signature sequences
+
+
+
Eukarya(-like) protein
synthesis
−
+
+
Cell wall made of
peptidoglycan
+
−
−
Cytoplasmic
membrane lipids
Fatty acids with ester
linkages
Long-chain, branched
hydrocarbons with
ether linkages
Fatty acids with ester
linkages
Sterols in membrane
− (some exceptions)
−
+
Nucleus and
membrane-bound
organelles
No
No
Yes
Flagellum
Bacterial flagellum
Archaellum
Eukaryotic flagellum
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Archaea
Eukarya
Multiple, linear
Chapter 5
Viral Structure and
Multiplication
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The Position of Viruses in the
Biological Spectrum
Viruses infect every type of cell, including bacteria,
algae, fungi, protozoa, plants, and animals
Seawater can contain 10 million viruses per milliliter
For many years, the cause of viral infections was
unknown:
• Louis Pasteur hypothesized that rabies was caused
by a “living thing” smaller than bacteria
• He also proposed the term virus, which is Latin for
“poison”
©McGraw-Hill Education
The Viral Debate
Two sides of the debate:
• Since viruses are unable to multiply independently
from the host cell, they are not living things and
should be called infectious molecules
• Even though viruses do not exhibit most of the life
processes of cells, they can direct them, and thus
are certainly more than inert and lifeless molecules
Viruses are better described as active or inactive
rather than alive or dead
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The Vital Role of Viruses in Evolution
Infect cells and influence their genetic makeup
Shape the way cells, tissues, bacteria, plants, and
animals have evolved
8% of the human genome consists of sequences
that come from viruses
10 to 20% of bacterial DNA contains viral sequences
Obligate intracellular parasites:
• Cannot multiply unless they invade a specific host cell
and instruct its genetic and metabolic machinery to
make and release new viruses
©McGraw-Hill Education
Properties of Viruses
(1)
Are obligate intracellular parasites of bacteria, protozoa, fungi,
algae, plants, and animals
Are ultramicroscopic in size, ranging from 20 nm up to 1,000 nm
(diameter)
Are not cells; structure is very compact and economical
Do not independently fulfill the characteristics of life
Basic structure consists of protein shell (capsid) surrounding
nucleic acid core
©McGraw-Hill Education
Properties of Viruses
(2)
Nucleic acid can be either DNA or RNA, but not both
Nucleic acids can be double-stranded DNA, single-stranded
DNA, single-stranded RNA, or double-stranded RNA
Molecules on virus surfaces give them high specificity for
attachment to host cell
Multiply by taking control of host cell’s genetic material and
regulating the synthesis and assembly of new viruses
Lack enzymes for most metabolic processes
Lack machinery for synthesizing proteins
©McGraw-Hill Education
How Viruses Are Classified and Named
For many years, animal viruses were classified on the
basis of their hosts and the diseases they caused
Newer classification systems emphasize the following:
• Hosts and diseases they cause
• Structure
• Chemical composition
• Similarities in genetic makeup
©McGraw-Hill Education
Virus Size Range
Smallest infectious agents
Smallest viruses: parvoviruses around 20 nm in
diameter
Largest viruses: herpes simplex virus around
150 nm in length
Some cylindrical viruses can be relatively long
(800 nm) but are so narrow in diameter (15 nm)
that their visibility is limited without an electron
microscope
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Size Comparison of Viruses with a
Eukaryotic Cell (Yeast) and Bacteria
Jump to long description
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Viral Architecture Is Best Observed with
Special Stains and Electron Microscopy
Jump to long description
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Source: CDCl/Dr. F. A. Murphy (a); ©Phototake (b); ©A.B. Dowsette/SPL/Science Source (c)
Viral Components
(1)
Viruses bear no resemblance to cells and lack any of
the protein-synthesizing machinery found in cells
Viral structure is composed of regular, repeating
subunits that give rise to their crystalline appearance
The structure contains only those parts needed to
invade and control a host cell:
• External coating
• Core containing one or more nucleic acid strains of DNA
or RNA
• Sometimes one or two enzymes
©McGraw-Hill Education
Viral Components
(2)
Capsid: protein shell that surrounds the nucleic acid:
• Nucleocapsid: the capsid together with the nucleic acid
• Naked viruses consist only of a nucleocapsid.
Envelope: external covering of a capsid, usually a
modified piece of the host’s cell membrane
Spikes can be found on naked or enveloped viruses:
• Project from the nucleocapsid or the envelope
• Allow viruses to dock with host cells
Virion: a fully formed virus that is able to establish an
infection in a host cell
©McGraw-Hill Education
Structure of Viruses
Jump to long description
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Viral Capsid
Capsid:
• Most prominent feature of viruses
• Constructed from identical protein subunits called
capsomeres
• Capsomeres spontaneously self-assemble into the
finished capsid
Two different types:
• Helical
• Icosahedral
©McGraw-Hill Education
Viral Envelope
Enveloped viruses:
• Take a bit of the cell membrane when they are
released from a host cell
Enveloped viruses can bud from:
• Cell membrane
• Nuclear envelope
• Endoplasmic reticulum
More flexible than the capsid so enveloped
viruses are pleomorphic
©McGraw-Hill Education
Helical Capsid Structure
Helical Capsids
Naked
Enveloped
The simpler helical capsids have rod-shaped capsomeres that bond together to
form a series of hollow discs resembling a bracelet. During the formation of the
nucleocapsid, these discs link with other discs to form a continuous helix into
which the nucleic acid strand is coiled.
The nucleocapsids of naked helical viruses are very rigid and tightly wound into a
cylinder-shaped package. An example is the tobacco mosaic virus, which attacks
tobacco leaves.
Enveloped helical nucleocapsids are more flexible and tend to be arranged as a
looser helix within the envelope. This type of morphology is found in several
enveloped human viruses, including influenza, measles, and rabies.
Naked Capsids
©McGraw-Hill Education
Enveloped Capsids
©Science Source, Source: CDC/Dr. Fred Murphy
Icosahedral Capsid Structure
Icosahedral
Capsids
These capsids form an icosahedron (eye″-koh-suh-hee′-drun)—a threedimensional, 20-sided figure with 12 evenly spaced corners. The arrangements of
the capsomeres vary from one virus to another. Some viruses construct the
capsid from a single type of capsomere, while others may contain several types
of capsomeres. There are major variations in the number of capsomeres; for
example, a poliovirus has 32, and an adenovirus has 252 capsomeres.
Naked
Adenovirus is an example of a naked icosahedral virus. In the photo you can
clearly see the spikes, some of which have broken off.
Enveloped
Two very common viruses, hepatitis B virus and the herpes simplex virus, possess
enveloped icosahedrons.
Naked Capsids
©McGraw-Hill Education
Enveloped Capsids
©Dr. Linda M. Stannard, University of Cape Town/Science Source, ©Dr. Linda M. Stannard, University of Cape Town/Science Source (hep B virus); ©Eye of Science/Science Source
Complex Capsid Structure
Complex
Capsids
©McGraw-Hill Education
Complex capsids, only found in the viruses that infect bacteria, may have
multiple types of proteins and take shapes that are not symmetrical. They are
never enveloped. The one pictured on the right is a T4 bacteriophage.
©AmiImages/Science Source
Nucleic Acids: At the Core of a Virus
Genome: the sum total of the genetic
information carried by an organism
Viruses contain DNA or RNA, but not both
The number of viral genes is quite small
compared with that of a cell:
• Four genes in hepatitis B virus
• Hundreds of genes in some herpesviruses
• Possess only the genes needed to invade host cells
and redirect their activity
©McGraw-Hill Education
Variety in Viral Nucleic Acid
DNA viruses: Single-stranded (ss) or double-stranded
(ds; linear or circular)
RNA viruses: can be double-stranded, but more often
single-stranded:
• Positive-sense RNA: ready for immediate translation
• Negative-sense RNA: must be converted before
translation can occur
• Segmented: individual genes exist on separate pieces of
RNA
• Retroviruses: carry their own enzymes to create DNA out
of their RNA
©McGraw-Hill Education
Viral Nucleic Acid
Virus Name
Disease It Causes
Variola virus
Smallpox
Herpes simplex II
Genital herpes
Parvovirus
Erythema infectiosum
(skin condition)
DNA Viruses Examples
Double-stranded DNA
Single-stranded DNA
RNA Viruses–Examples
Single-stranded (+) sense Poliovirus
Poliomyelitis
Single-stranded (−) sense Influenza virus
Influenza
Double-stranded RNA
Rotavirus
Gastroenteritis
Single-stranded RNA +
reverse transcriptase
HIV
AIDS
©McGraw-Hill Education
Other Substances in the
Virus Particle
Enzymes for specific operations within their host cell:
• Polymerases that synthesize DNA and RNA
• Replicases that copy RNA
• Reverse transcriptase synthesizes DNA from RNA
Completely lack the genes for synthesis of metabolic
enzymes
Some viruses carry away substances from their host cell:
• Arenaviruses pack along host ribosomes
• Retroviruses borrow the host’s tRNA molecules
©McGraw-Hill Education
Lytic Replication Cycle
in Animal Viruses
General phases of the animal lytic viral replication
cycle:
• Adsorption(Attachment)
• Penetration
• Uncoating
• Synthesis
• Assembly
• Release
The length of the replication cycle varies from 8
hours in polioviruses to 36 hours in herpesviruses
©McGraw-Hill Education
Adsorption(Attachment)
A virus can invade its host cell only through making
an exact fit with a specific host molecule
Host range: the limited range of cells that a virus
can infect:
• Hepatitis B: liver cells of humans
• Poliovirus: intestinal and nerve cells of primates
• Rabies: various cells of all mammals
Cells that lack compatible virus receptors are
resistant to adsorption and invasion by that virus
Tropisms: specificities of viruses for certain tissues
©McGraw-Hill Education
Viral Attachment Process
Jump to long description
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Penetration and Uncoating
The flexible cell membrane of the host is
penetrated by the whole virus or its nucleic acid
Penetration through endocytosis happens when an
entire virus is engulfed by the cell and enclosed in a
vacuole or vesicle
Direct fusion of the viral envelope with the host
cell membrane:
• Envelope merges directly with the cell membrane,
liberating the nucleocapsid into the cell’s interior
©McGraw-Hill Education
Penetration by Animal Viruses
Jump to long description
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Synthesis: Replication and
Protein Production
DNA viruses:
• Enter the host cell’s nucleus and are replicated and
assembled there
RNA viruses:
• Replicated and assembled in the cytoplasm
Retroviruses turn their RNA genomes into DNA
©McGraw-Hill Education
Assembly and Release
Assembly: virus is put together using “parts”
manufactured during the synthesis process
Release: the number of viruses released by
infected cells is variable, controlled by:
• Size of the virus
• Health of the host cell
Poxvirus-infected cell: 3,000 to 4,000 virions
Poliovirus-infected cell: 100,000 virions
Immense potential for rapid viral proliferation
©McGraw-Hill Education
Maturation and Release of
Enveloped Viruses
©McGraw-Hill Education
©Chris Bjornberg/Science Source (b)
Lytic Cycle of Animal Viruses
(1)
1. Adsorption(Attachment)
• The virus encounters a susceptible host cell and adsorbs specifically
to receptor sites on the cell membrane
• The membrane receptors that viruses attach to are usually proteins
that the cell requires for its normal function
• Glycoprotein spikes on the envelope (or on the capsid of naked
viruses) bind to the cell membrane receptors
2. Penetration and Uncoating
• In this example, the entire virus is engulfed (endocytosed) by the cell
and enclosed in a vacuole or vesicle
• When enzymes in the vacuole dissolve the envelope and capsid, the
virus is said to be uncoated, a process that releases the viral nucleic
acid into the cytoplasm
©McGraw-Hill Education
Life Cycle of Animal Viruses
(2)
3. Synthesis: Replication and Protein Production
• Viral nucleic acid begins to synthesize the building blocks for new
viruses
• Some viruses come equipped with the necessary enzymes for
synthesis of viral components; others utilize those of the host
• Proteins for the capsid, spikes, and viral enzymes are synthesized on
the host’s ribosomes using its amino acids
©McGraw-Hill Education
Life Cycle of Animal Viruses
(3)
4. Assembly
•
Mature virus particles are constructed from the growing pool of parts
•
Capsid is first laid down as an empty shell that will serve as a receptacle for
the nucleic acid strand
•
Viral spikes are inserted into the host’s cell membrane so they can be picked
up as the virus buds off with its envelope
5. Release
•
Assembled viruses leave their host in one of two ways:
•
Nonenveloped and complex viruses that reach maturation in the cell nucleus or cytoplasm are
released when the cell lyses or rupture
•
Enveloped viruses are liberated by budding from the membranes of the cytoplasm, nucleus,
endoplasmic reticulum, or vesicles
•
During this process, the nucleocapsid binds to the membrane, which curves
completely around it and forms a small pouch
•
Pinching off the pouch releases the virus with its envelope
©McGraw-Hill Education
Lysogenic Cycle
Persistent Infections
Some cells maintain a carrier relationship: cell harbors
the virus and is not immediately lysed:
• Can last from a few weeks to the remainder of the host’s life
• Can remain latent in the cytoplasm
Provirus:
• Viral DNA incorporated into the DNA of the host
• Measles virus
Chronic latent state:
• Periodically become activated under the influence of various
stimuli
• Herpes simplex and herpes zoster viruses
©McGraw-Hill Education
Damage to the Host Cell
Cytopathic effects (CPEs): virus-induced damage to the
cell that alters its microscopic appearance
Types of CPEs include:
• Gross changes in shape and size
• Development of intracellular changes
• Inclusion bodies: compacted masses of viruses or
damaged cell organelles in the nucleus and cytoplasm
• Syncytia: fusion of multiple damaged host cells into
single large cells containing multiple nuclei (giant cells)
Accumulated damage from a virus infection kills most
host cells
©McGraw-Hill Education
Cytopathic Changes
Jump to long description
©McGraw-Hill Education
Source: CDC (a); Courtesy Massimo Battaglia, INeMM CNR, Rome Italy (b)
Viruses and Cancer
(1)
Experts estimate that 13% of cancers are caused
by viruses
Transformation: the effect of oncogenic, or
cancer-causing viruses:
• Some viruses carry genes that directly cause cancer
• Other viruses produce proteins that induce a loss of
growth regulation, leading to cancer
©McGraw-Hill Education
Viruses and Cancer
(2)
Transformed cells:
• Increased rate of growth
• Changes in their chromosomes
• Changes in cell’s surface molecules
• Capacity to divide indefinitely
Oncoviruses: mammalian viruses capable of initiating
tumors:
• Papillomaviruses
• Herpesviruses
• Hepatitis B virus
• HTLV-I
©McGraw-Hill Education
Viruses That Infect Bacteria
Bacteriophage: “bacteria eating”:
• Most contain double-stranded DNA, but some RNA
types exist as well
• Every bacterial species is parasitized by various
specific bacteriophages
• The bacteria they infect are often more pathogenic
for humans
©McGraw-Hill Education
T-Even Bacteriophage
Infect E. coli
Structure:
• Icosahedral capsid
containing DNA
• Central tube
surrounded by a sheath
• Collar
• Base plate
• Tail pins
• Fibers
Jump to long description
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Events in the Lytic Cycle of
T-even Bacteriophages
(1)
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Events in the Lytic Cycle of
T-even Bacteriophages
(2)
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Lysogenic Cycle:
The Silent Virus Infection
Temperate phages:
•
•
Undergo adsorption and penetration
Do not undergo replication or release immediately
Viral DNA enters an inactive prophage state:
•
•
•
Inserted into bacterial chromosome
Copied during normal bacterial cell division
Lysogeny: a condition in which the host chromosome
carries bacteriophage DNA
Induction: prophage in a lysogenic cell becomes
activated and progresses directly into viral
replication and the lytic cycle
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The Role of Lysogeny
in Human Disease
Occasionally, phage genes in the bacterial
chromosome cause the production of toxins or
enzymes that the bacterium would not
otherwise have
Lysogenic conversion: when a bacterium
acquires a new trait from its temperate phage:
• Corynebacterium diphtheriae – diphtheria toxin
• Vibrio cholerae – cholera toxin
• Clostridium botulinum – botulinum toxin
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Techniques in Cultivating and
Identifying Animal Viruses
Viruses require living cells as their “medium”:
• In vivo: laboratory-bred animals and embryonic bird
tissues
• In vitro: cell or tissue culture methods
Primary purposes of viral cultivation:
• Isolate and identify viruses in clinical specimens
• Prepare viruses for vaccines
• Do detailed research on viral structure, multiplication
cycles, genetics, and effects on host cells
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Using Live Animal Inoculation
Specially bred strains of white mice, rats,
hamsters, guinea pigs, and rabbits are the usual
choices for viral cultivation
Occasionally, invertebrates such as insects or
nonhuman primates are used
Because viruses exhibit host specificity, certain
animals can propagate viruses more readily than
others
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Using Bird Embryos
Bird eggs containing embryos:
• Intact and self-supporting unit
• Sterile environment
• Contain their own nourishment
Chicken, duck, and turkey eggs are the most
common choices for inoculation
Viruses are injected through the eggshell by
drilling a small hole or making a small window
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Chicken Egg Used to Culture a Virus
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Using Cell (Tissue)
Culture Techniques
Isolated animal cells are grown in vitro in cell or
tissue culture rather than in an animal or egg
Cell culture, or tissue culture:
• Grown in sterile chambers with special media that
contain the correct nutrients for cells to survive
• Cells form a monolayer, or single, confluent sheet of
cells that supports viral multiplication
• Allows for the close inspection of culture for signs of
infection
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Detection of Viral Growth
in Culture
Observation of degeneration and lysis of
infected cells
Plaques: areas where virus-infected cells have
been destroyed show up as clear, well-defined
patches in the cell sheet:
• Visible manifestation of cytopathic effects (CPEs)
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Normal and Infected Cell Culture
Jump to long description
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Source: Bakonyi T, Lussy H, Weissenböck H, Hornyák A, Nowotny N. Emerging Infectious Diseases, Vol. 11, No. 2, Feb. 2005.
Viroids
Virus-like agents that parasitize plants
About one-tenth the size of an average virus
Composed of naked strands of RNA, lacking a
capsid or any other type of coating
Significant pathogens in economically important
plants: tomatoes, potatoes, cucumbers, citrus
trees, chrysanthemums
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Potato Infected With a Virus
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